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This lesson covers translation — the second stage of gene expression in which the nucleotide sequence of mRNA is decoded to produce a polypeptide. Understanding translation is essential for the Edexcel A-Level Biology specification (9BI0, Topic 7).
Translation is the process by which the sequence of codons on mRNA is decoded by ribosomes and transfer RNA (tRNA) to assemble a chain of amino acids — a polypeptide. Translation takes place on ribosomes in the cytoplasm (or on the rough endoplasmic reticulum for secreted proteins).
Ribosomes are the molecular machines that carry out translation. They consist of two subunits made of ribosomal RNA (rRNA) and proteins:
| Feature | Eukaryotic ribosome | Prokaryotic ribosome |
|---|---|---|
| Size | 80S | 70S |
| Large subunit | 60S | 50S |
| Small subunit | 40S | 30S |
Each ribosome has three binding sites for tRNA:
tRNA molecules are small RNA molecules (about 75–90 nucleotides long) with a distinctive cloverleaf secondary structure that folds into an L-shaped three-dimensional structure.
Key features of tRNA:
| Feature | Function |
|---|---|
| Anticodon | A triplet of bases complementary to the mRNA codon; ensures the correct amino acid is delivered |
| Amino acid attachment site | The 3' end (CCA sequence) where the specific amino acid is covalently attached |
| Aminoacyl-tRNA synthetase | The enzyme that attaches the correct amino acid to its corresponding tRNA |
There are at least 20 different aminoacyl-tRNA synthetases — one for each amino acid. Each enzyme recognises both the amino acid and its correct tRNA, ensuring accurate translation. This step requires ATP.
Exam Tip: The specificity of aminoacyl-tRNA synthetases is crucial for the accuracy of translation. If the wrong amino acid is attached to a tRNA, the wrong amino acid will be incorporated into the polypeptide regardless of correct codon-anticodon pairing.
Elongation is a cyclic process with three steps per cycle:
Step 1: Codon recognition
Step 2: Peptide bond formation
Step 3: Translocation
This cycle repeats as the ribosome reads each codon in sequence.
Multiple ribosomes can translate the same mRNA molecule simultaneously. This structure is called a polyribosome or polysome. Each ribosome produces its own copy of the polypeptide, so a single mRNA can be used to produce many copies of the same protein efficiently.
After translation, the polypeptide often undergoes further modification before becoming a functional protein:
| Modification | Description |
|---|---|
| Folding | The polypeptide folds into its secondary, tertiary and (if applicable) quaternary structure, often assisted by chaperone proteins |
| Cleavage | Removal of the initial methionine or signal peptides |
| Glycosylation | Addition of carbohydrate groups (in the Golgi apparatus) |
| Phosphorylation | Addition of phosphate groups by kinase enzymes |
| Disulfide bond formation | Covalent bonds between cysteine residues stabilise tertiary structure |
Exam Tip: Remember that the primary structure (sequence of amino acids) determines all higher levels of protein structure. Any change to the amino acid sequence — caused by a mutation — may alter the protein's three-dimensional shape and therefore its function.
Translation is an energy-demanding process:
| Step | Energy source |
|---|---|
| Amino acid activation (charging tRNA) | ATP → AMP + PPi |
| Codon recognition (A site binding) | GTP |
| Translocation | GTP |
Producing a single polypeptide of 100 amino acids requires approximately 400 high-energy phosphate bonds.
| Feature | Transcription | Translation |
|---|---|---|
| Template | DNA (template strand) | mRNA |
| Product | mRNA | Polypeptide |
| Location (eukaryotes) | Nucleus | Cytoplasm / rough ER |
| Enzyme | RNA polymerase | Ribosome (peptidyl transferase) |
| Building blocks | RNA nucleotides (A, U, G, C) | Amino acids |
| Direction of reading | 3' → 5' (template strand) | 5' → 3' (mRNA) |
| Energy | ATP (NTPs) | ATP and GTP |
Several antibiotics work by inhibiting prokaryotic translation, exploiting differences between 70S and 80S ribosomes:
| Antibiotic | Mechanism |
|---|---|
| Tetracycline | Blocks tRNA binding to the A site |
| Chloramphenicol | Inhibits peptidyl transferase |
| Erythromycin | Blocks translocation |
| Streptomycin | Causes misreading of mRNA |
These antibiotics target prokaryotic 70S ribosomes and do not affect eukaryotic 80S ribosomes, making them useful for treating bacterial infections without harming human cells.
| Stage | Key Events |
|---|---|
| Initiation | Small subunit + mRNA + initiator tRNA → complete ribosome at start codon |
| Elongation | Codon recognition → peptide bond formation → translocation (repeated) |
| Termination | Stop codon reached → release factor → polypeptide released |
| Post-translational | Folding, cleavage, glycosylation, phosphorylation |
Exam Tip: When describing translation, always mention the roles of mRNA (carries the genetic code), tRNA (carries amino acids and has anticodon), and ribosomes (provide the site for polypeptide assembly and catalyse peptide bond formation). Use the terms codon and anticodon correctly.
This material sits in Edexcel 9BI0 Topic 8 (Grey Matter — Coordination, Response and Gene Technology), which expects candidates to describe translation as the ribosomal decoding of mature mRNA into a polypeptide using tRNA adaptors, codon–anticodon pairing, peptidyl transferase activity of the large subunit (the ribosome as a ribozyme), the A, P, E sites as the geometric framework of elongation, and the start (AUG) and stop (UAA, UAG, UGA) codons as the punctuation of the message. Synoptic links run backwards to lesson 2 on transcription (which produces the mRNA translated here); to lesson 1 on the genetic code (codon table, redundancy, wobble); to Topic 1 (protein structure) for primary–quaternary levels (translation outputs only primary sequence); to Topic 2 (cell biology) for free cytoplasmic vs rough ER ribosomes (the latter recognised by an N-terminal signal peptide and translated co-translationally into the ER lumen); and to Topic 6 (Infection, Immunity and Forensics) for antibiotic selectivity at bacterial 70S vs eukaryotic 80S ribosomes — with mitochondrial 70S ribosomes (endosymbiotic relic) explaining certain drug toxicities. Refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks):
(a) Describe the events of translation in a eukaryotic cell, from binding of the small ribosomal subunit to release of the completed polypeptide. (4)
(b) The mature mRNA produced from a gene begins 5'-AUG GCA UUU UAA-3'. State (i) the anticodon of the initiator tRNA, (ii) the sequence of amino acids in the polypeptide produced, and (iii) one reason why a tRNA-Ala with anticodon 3'-CGI-5' can read more than one codon for alanine. (4)
Solution with mark scheme:
(a) M1 (AO1) — initiation. The small (40S) ribosomal subunit, recruited by the 5' cap of the mature mRNA, scans along the 5' UTR until it locates the start codon (AUG). The initiator tRNA-Met, with anticodon 3'-UAC-5' (or, written 5' to 3', 5'-CAU-3'), binds AUG by complementary antiparallel base pairing and occupies the P site. The large (60S) subunit then joins, forming the complete 80S ribosome.
A1 (AO1) — elongation. A second charged tRNA, with an anticodon complementary to the next codon, enters the A site (this step consumes GTP). The ribosome's peptidyl transferase activity — an rRNA ribozyme function of the large subunit — catalyses the formation of a peptide bond between the methionine in the P site and the amino acid in the A site, transferring the dipeptide to the A-site tRNA.
A1 (AO1) — translocation. The ribosome moves one codon along the mRNA in the 5'→3' direction (consuming GTP). The dipeptidyl-tRNA shifts from A to P; the now-deacylated initiator tRNA shifts from P to E (exit) and dissociates; the next codon is exposed in the empty A site. The cycle repeats.
A1 (AO1) — termination. When the ribosome reaches a stop codon (UAA, UAG or UGA) in the A site, no tRNA has a complementary anticodon. Instead, a release factor binds the stop codon, triggers hydrolysis of the bond between the polypeptide and the final tRNA, and the completed polypeptide is released. The ribosomal subunits, mRNA and release factor dissociate.
(b) M1 (AO2) — anticodon of initiator tRNA. The initiator tRNA reads AUG. Its anticodon is antiparallel and complementary: written 3'→5' it is 3'-UAC-5', equivalently 5'-CAU-3'.
A1 (AO2) — polypeptide sequence. The codons are AUG | GCA | UUU | UAA. AUG = methionine; GCA = alanine; UUU = phenylalanine; UAA = stop. The polypeptide is therefore Met–Ala–Phe (the methionine is often subsequently cleaved by methionyl aminopeptidase).
A1 (AO3.1) — wobble. The third base of the anticodon (here inosine, I) can pair with U, C or A in the third position of the mRNA codon — the wobble rule. The codons GCU, GCC and GCA all code for alanine, so a single tRNA-Ala with anticodon 3'-CGI-5' decodes all three.
A1 (AO3.2) — extension. Wobble reduces tRNA inventory: ~30–40 tRNAs decode all 61 sense codons, and silent point mutations at the third codon position are buffered.
Total: 8 marks (M2 A6).
Question (6 marks): A scientist measured the rate of polypeptide synthesis in two systems. In system X, streptomycin (an antibiotic) was added; in system Y, cycloheximide (a different antibiotic) was added. The mRNA in both systems coded for a 300-amino-acid polypeptide of human origin, but the ribosomes were of bacterial origin (70S) in system X and of eukaryotic origin (80S) in system Y. Polypeptide yield fell to near zero in system X but was unchanged in system Y.
Using the data, explain the result and discuss what it shows about the use of antibiotics in human medicine.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Stating that streptomycin targets 70S (prokaryotic) ribosomes by causing misreading of mRNA / blocking initiation |
| 2 | AO1.2 | Stating that 70S and 80S ribosomes differ in subunit composition (50S+30S vs 60S+40S) and rRNA sequence |
| 3 | AO2.1 | Linking the fall in yield in system X to streptomycin's selective binding to 70S (bacterial) ribosomes |
| 4 | AO2.7 | Explaining that system Y is unaffected because cycloheximide targets 80S but is irrelevant here, OR that 80S ribosomes lack the streptomycin binding site |
| 5 | AO3.1 | Concluding that selective toxicity — drug binds bacterial but not human ribosomes — is the basis of antibiotic safety |
| 6 | AO3.2 | Justifying why mitochondrial ribosomes (70S) can be a source of side effects (e.g. aminoglycoside ototoxicity) — endosymbiotic origin |
Total: 6 marks (AO1 = 2, AO2 = 2, AO3 = 2). Edexcel reliably tests translation through "explain why an antibiotic kills bacteria but not the patient" prompts; candidates who name 70S/80S without naming the mechanistic difference (rRNA sequence, drug binding site, mitochondrial ribosome exception) lose AO3 marks.
Lesson 2 (transcription) — the mature mRNA is the substrate for translation. The 5' cap recruits the small subunit; the 5' UTR is scanned for AUG; the 3' poly-A tail recruits poly-A binding proteins that loop the mRNA into a closed-loop topology. Translation cannot occur on pre-mRNA — introns must first be removed.
Lesson 1 (genetic code) — the codon table is decoded here. AUG = start = methionine (also internal Met); UAA/UAG/UGA = stop (no tRNA — release factor); degeneracy is realised biochemically through the wobble rule, allowing ~30–40 tRNAs to decode 61 sense codons.
Topic 1 (protein structure) — translation produces primary structure only. The output is a linear chain of amino acids joined by peptide bonds. Secondary (α-helices, β-sheets), tertiary (R-group interactions: H-bonds, ionic, hydrophobic, disulfide bridges) and quaternary (multi-subunit, e.g. haemoglobin α₂β₂) structures form post-translationally, often assisted by chaperones (Hsp70, Hsp60/GroEL). Misfolding causes Alzheimer's, Parkinson's, prion diseases.
Topic 2 (cells — free vs ER ribosomes). Free cytoplasmic ribosomes make cytosolic, nuclear, mitochondrial and chloroplast proteins. Rough ER ribosomes make secreted, lysosomal and membrane proteins. The decision is co-translational: an N-terminal signal peptide is recognised by the signal recognition particle (SRP), which docks the ribosome to the ER membrane; translation proceeds into the ER lumen, the signal peptide is cleaved, and the protein enters the secretory pathway (ER → Golgi → vesicle → plasma membrane).
Topic 6 (antibiotics and 70S ribosomes). Bacterial ribosomes are 70S (50S + 30S); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S). Antibiotics bind selectively: tetracycline blocks the A site; chloramphenicol inhibits peptidyl transferase; erythromycin blocks translocation; streptomycin causes 30S misreading. Mitochondrial ribosomes are also 70S — an endosymbiotic relic — which explains aminoglycoside ototoxicity.
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